1. Field of the Invention
The present invention relates generally to semiconductor fabrication. More particularly, the present invention relates to the process of photolithography, and even more specifically to baking a photoresist onto a substrate, such as a photomask, during the photolithography process.
2. Background of the Related Art
This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
During fabrication of a semiconductor device, a process called photolithography is employed to create patterns on a semiconductive substrate, commonly known as a wafer. The wafer is built up one layer at a time creating an overlay of complex patterns which ultimately form electrical devices and paths. The photolithography process comprises a variety of steps to accomplish the patterning of each layer. One common step of the process involves the use of one or more photomasks. Each photomask has a pattern formed thereon, and this pattern is transferred onto a semiconductor wafer by irradiating the wafer through the photomask.
To fabricate a photomask, surface preparation is required wherein a surface of the substrate, often referred to as a photomask blank, is cleaned and dried. The substrate may be composed any of a number of materials. For example, the photomask blank may be made of glass or quartz. Surface preparation of the substrate is done in anticipation of a photoresist being applied to the substrate's surface. The photoresist requires a clean and dry surface in order to accomplish a high level of adhesion. The photoresist is typically applied by placing a coat of the material on the substrate and then spinning the substrate to obtain a thin uniform film across the blank's surface. After the photoresist has been applied, the blank and film are subjected to a first baking process often referred to as soft bake or pre-bake. The soft bake process serves to evaporate a portion of the photoresist solvents. Besides removing unwanted solvents, which will interfere with subsequent processing, the soft bake also helps to facilitate better adhesion between the photoresist and the substrate. After the soft bake the photoresist remains as a relatively soft coating on the substrate.
The baking process may be accomplished using various methods. Two of the more typical methods include use of a hot plate or the use of a convection oven. In the hot plate method, the substrate is placed directly on the hot plate for heating by conduction. Heat is transferred from the hot plate to the photomask blank and then through the blank to the photoresist layer. This technique provides good temperature control and allows for small batches to be processed simultaneously.
Alternatively, the convection oven method utilizes a fluid medium, usually a gas such as air, to heat the substrate and film. Convection baking allows for a more direct baking of the photoresist layer since convection baking does not have to rely on conduction through the substrate. Convection ovens permit large batches of photomasks to be processed at one time, but typically these ovens have inferior temperature control in comparison with the hot plate method. Convection ovens also typically take longer per batch to process than do hot plates. Other alternative methods for baking include microwave, infra-red, and vacuum oven baking.
Various sources of exposure, including optical sources, x-rays or ion beams, may be used for exposing the photoresist. The exposure causes a chemical reaction to take place in the photoresist layer. For example, in one type of photoresist, exposure causes a polymerization of the photoresist. Thus, by using a mask and an exposure source, a pattern of polymerized resist (and a mating pattern of non-polymerized resist) is formed on the surface of the wafer. This process, while described above in general terms, is actually rather complex and involved. Likewise, there are various exposure sources to choose from, each with its own advantages and complexities. Also, there are multiple types of photoresist. Each type of photoresist has different characteristics and responds differently to the various manipulative steps in the photolithography process.
After the film of photoresist has been exposed, the photoresist is then developed. Developing is a chemical process wherein chemical dissolution of unpolymerized regions in the photoresist occurs. Different developing chemicals and techniques are often employed depending on the type of photoresist being used. After the photoresist has been developed, the chemical is rinsed off and the substrate is allowed to dry. Polymerized regions of the photoresist remain on the surface of the substrate. After developing of the photoresist, the substrate may optionally undergo a second baking process. The second baking process, often referred to as hard bake, again serves to evaporate remaining solvents in the photoresist and to create better adhesion of the photoresist to the substrate. The methods and techniques used for hard baking are essentially the same as those used for soft baking.
While described in generalities above, the process for fabricating photomasks is complex and requires careful attention to many details. Mistakes and errors can be introduced at any step of the process causing resultant defects in the final product. Likewise, each step of the process is continually scrutinized for possible improvements. One area where improvement is contemplated is in the baking processes. A good deal of variability may be introduced into the process during the baking steps. For example, it has been noted that the temperature gradient found in a substrate during baking is not uniform. This means that the temperature at the outside edge of the substrate is not the same as the temperature at the center of the substrate. Often the range of the temperature gradient is several degrees. The variation of temperature results in uneven baking of the photoresist layer. The uneven baking can lead to poor performance of the photoresist layer during the exposure and developing steps. For example, the lines formed in the photoresist layer during exposure and developing can vary in width depending on their location on the photomask blank. A region of the photoresist layer baked at the desired temperature will produce lines at a predicted width, however, a region of the photoresist layer baked at the varied temperature will produce lines which vary from the predicted width. Thus the precision of the exposed image becomes a function of the temperature gradient experienced by the photoresist layer.
In consideration of the heating methods employed, numerous factors affect the resultant baked film. For example, one problem with the hot plate method of baking is that the surface of the hot plate may not be co-incidentally parallel with the surface of the substrate being baked. The result of the two surfaces not being exactly parallel is air gaps present between the two surfaces. Since the hot plate method of baking is a process of conduction, the air gaps create an inefficiency because localized regions experience heat transfer by convection instead of conduction. The conduction transfers the heat to the substrate much quicker than does the convection in the air gaps. Therefore, heat is unevenly distributed to the surface of the substrate from the hot plate. Again the ultimate result is an undesired temperature gradient in the substrate and non-uniform baking of the photoresist layer.
Another problem associated with hot plate baking is the transient formation of temperature zones in the hot plate. This is often the result of a rapid temperature spike in the heating element. The heating element attains a specific temperature and then the surrounding material tries to attain the same temperature as the heating element. Simply stated, the hot plate is trying come to an equilibrium temperature, but in the process temperature zones are created. These temperature zones are transient, but can result in similar temperature zones being transferred to the substrate and film. As an example, on study has found that, depending on the particular steps and methods followed, hot plate baking may result in temperature variations of from 3° to 6° C. over a 132 mm square area.
While convection ovens generally do a better job in respect to minimizing the production of temperature zones, there are opportunities for improvement. For example, the medium used for heat transfer in convection ovens is typically air. Convection by air is not as efficient as the hot plate method of conduction. The relative inefficiency of air as heat transfer medium is one reason why the convection method is typically slower than the hot plate method.
The present invention may address one or more of the problems set forth above.